This invention relates to monolithic aerogel catalysts and composite materials, specifically to the use of ultra-thin aerogel honeycomb catalyst monoliths for selective catalytic reaction of gas phase chemical species.
Aerogel catalysts are generally used in the form of fine powders or lumps which are fragile, loose, and difficult to handle in chemical reactors. Severe pressure drops and heat and mass transfer limitations occur in fixed bed reactors where Aerogels are used in these types of physical forms. Other alternative forms such as Aerogel coatings on Rashig rings or Aerogels being embedded into alundum boiling stones have been tried with limited success to assist in improving on the above limitations. Fluidized bed reactors have also been piloted using the xe2x80x9clumpsxe2x80x9d form of Aerogels with limited success.
European Patent Number #0186149 by Stauffer Chemical Company describes the preparation of non-aged, inorganic oxide containing aerogels. The method comprises the steps of dissolving the alkoxide in a solvent, optionally adding a catalytic amount of a base or acid, and hydrolyzing the metal compound to produce a gel. The solvent in the gel is exchanged with an extraction fluid, and the fluid in the gel is supercritically extracted to form an aerogel. The patent describes the preparation of amorphous, granular metal oxide aerogels, rather than monolithic forms.
Transparent Metal Oxide Aerogel Monoliths have been successfully formed by Lawrence Livermore National Laboratory, U.S. Pat. No. 5,395,805 to Droege (1995), in samples approximately 1 inch in diameter and 0.25 inches thick. This type of small monolith has extremely limited commercial catalytic applications due to its essentially inaccessible internal surface area. The pressure drop that is required to access the internal surface area is tremendously high. Per the LLNL patent, the fabrication of these small monoliths requires a containment vessel that is sealed in such a way as to be gas permeable.
Conventional Honeycomb Monolith Chemical Reaction beds for NOx reduction are typically at least 20 feet in depth (a 20 foot superficial gas flow path) and have the disadvantages of relatively high pressure drop, laminar flow in the honeycomb channels, and active catalyst surface limited to the surface washcoating of the catalyst impregnated on a ceramic honeycomb monolith.
Current catalyst pore structures depend on the micropore and macropore structure of the material of the base monolith and the ability to uniformly apply a washcoat of material over the monolith. Washcoat connections with the support via thin branches in small pores are highly vulnerable to thermal stress cracking. Typical internal surface areas for a Titania monolith are approximately 50 M2 per gram of material. The washcoat layer surface area is normally in the range of 100 to 200 M2 per gram of material. Once a thin washcoat has been poisoned by materials such as alkalies and sulfur oxides, the catalyst will be deactivated.
A conventional composition for a NOx reduction catalyst that utilizes ammonia for its reduction agent is in the range of four to eight weight percent Vanadium Oxide or Tungsten Oxide coated over a Titania monolith. The current commercial catalysts have a formulation tradeoff limitation between more Vanadium which increases the activity toward NOx reduction but also increases the activity of the unwanted oxidation reaction of SO2 to SO3. SO3 combines with the ammonia to form ammonium bi-sulfate or ammonium sulfate which can cause corrosion and plugging of the downstream heat exchange equipment. The Vanadium Oxide allows activity toward NOx in lower operating temperature zones than the Tungsten Oxide.
Aerogel Matrix composites using fibers dispersed within the bulk aerogel have been successfully formed by Battelle Memorial Institute U.S. Pat. No. 5,306,555 to Ramamkurthl (1994). These samples were formed with a high weight percentage of fibers, from 9 to 35, and had relatively low surface areas from 147 to 303 M2 per gram of material.
Although these related patents discuss the formulation of metal oxide Aerogels and methods of fabrication of small Arogel monoliths over long time periods (days), none address the practical application of Aerogels as catalysts. Economic fabrication techniques for Aerogel catalyst sections where the inherently large internal surface area characteristics can be fully exploited at low pressure drops in gas reacting systems are not addressed. The present invention addresses the need for a catalyst that allows Selective Catalytic Reduction of NOx by using large Ultra-Thin Honeycomb Aerogel catalyst sections that allow the unique surface area of Aerogels to be fully exploited at very low gas pressure drops.
The following lists several Objects and Advantages of the invention:
(a) The Ultra-Thin Aerogel Honeycomb Monolith has flow through gas channels that will allow the chemical reactants to access the entire internal surface area of the Aerogel Catalyst with minimal pressure drop.
(b) Space velocities are nearly an order of magnitude greater than conventional catalysts, this allows for extremely thin catalyst sections to be effectively utilized.
(c) The Ultra-Thin catalyst crossection will allow a much greater effective mass transfer coefficient.
(d) The combination of advantages (b) and (c) will allow the reactor catalyst section to be more highly selective toward the reduction of NOx versus the unwanted side reaction of oxidation of SO2 to SO3.
(e) The Aerogel catalyst will have much greater life than conventional washcoated catalyst due to its homogeneous nature, and the entire monolith surface being chemically reactive.
(l) Zirconium was added to the Transition Metal Oxides mix of Vanadium and Tungsten to create site dislocations in the crystalline structure of the catalytic surface and thereby increase reactive surface area.